This invention relates to electrolytic separation of metals and, more particularly, to an improved method for selectively removing an external tin layer of a copper or copper alloy substrate by controlled potential electrolysis utilizing a particular electrolyte solution.
Copper and copper alloy substrates are frequently tinned using various techniques to form metal composites, including hot dipping, electroplating, etc. so as to provide improved solderability shelf life and longer shelf life with respect to low contact resistance. Both during the composite forming processes and in fabrication of metal products from the metal composite stock, some amount of off-grade scrap composite metal is inevitably produced. It is highly desirable in order to conserve materials and to reduce costs to recover the components of the scrap composite and to reuse these components in further operations. Scrap from tinned copper and copper alloy substrates is frequently difficult to handle in a brass mill.
In certain instances, scrap can be consumed by remelting into tin containing alloys such as the phosphor bronzes and tin brasses. However, when the substrate alloy contains other alloying additions not permitted within the compositional range of the two above alloy families, then this most economical route for consumption of the tin scrap does not exist. Alloy C66400 containing nominally 11% zinc with iron and cobalt additions is a good example of this. Neither the zinc, iron nor cobalt can be tolerated within the impurity limits of the phosphor bronzes. The iron and cobalt cannot be tolerated within the compositional limits of the tin brasses except in cases where the tin scrap comprises only a small portion of the overall melt.